st: randomly assign btw datasets

I want to generate a random sample of combinations between a group of firms
(#=100,000) and a list of industry SICs (#=50) (in two datasets). I could
use cross to generate all possible combinations and then random sample.
Because the resulting dataset would be huge, I decide to do it another way:
random sample the # of firms (say, 30,000), and then randomly assign one SIC
to each sampled firm. How can I do it? Thanks for your help.